Contour transform options#

This example demonstrates the difference between transforming the points before/after generating the contours. It uses the transform_first keyword argument to indicate that Cartopy should transform the points before calling the contouring algorithm, which can have a significant impact on speed (it is much faster to transform points than it is to transform patches). This does have a negative impact on the wrapped coordinates as one can see in the second axes that the data does not extend to the full global extent.

transform_first=False, transform_first=True
import matplotlib.pyplot as plt

import cartopy.crs as ccrs


def sample_data(shape=(73, 145)):
    """Return ``lons``, ``lats`` and ``data`` of some fake data."""
    import numpy as np

    nlats, nlons = shape
    lats = np.linspace(-np.pi / 2, np.pi / 2, nlats)
    lons = np.linspace(0, 2 * np.pi, nlons)
    lons, lats = np.meshgrid(lons, lats)
    wave = 0.75 * (np.sin(2 * lats) ** 8) * np.cos(4 * lons)
    mean = 0.5 * np.cos(2 * lats) * ((np.sin(2 * lats)) ** 2 + 2)

    lats = np.rad2deg(lats)
    lons = np.rad2deg(lons)
    data = wave + mean

    return lons, lats, data


def main():

    # Use the same sample data as the waves example, but make it
    # more dependent on y for more interesting contours.
    x, y, z = sample_data((20, 40))
    z = z * -1.5 * y

    # Setup a global EckertIII map with faint coastlines.
    fig = plt.figure()
    ax1 = fig.add_subplot(2, 1, 1, projection=ccrs.EckertIII())
    ax1.set_title("transform_first=False")
    ax2 = fig.add_subplot(2, 1, 2, projection=ccrs.EckertIII())
    ax2.set_title("transform_first=True")

    for ax, transform_first in zip([ax1, ax2], [False, True]):
        ax.set_global()
        ax.coastlines('110m', alpha=0.1)

        # Add colourful filled contours.
        filled_c = ax.contourf(x, y, z, transform=ccrs.PlateCarree(),
                               transform_first=transform_first)

        # And black line contours.
        ax.contour(x, y, z, levels=filled_c.levels,
                   colors=['black'],
                   transform=ccrs.PlateCarree(),
                   transform_first=transform_first)

    plt.show()


if __name__ == '__main__':
    main()

Total running time of the script: ( 0 minutes 0.283 seconds)

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